Conventional optical photolithography is used to fabricate surface structures on semiconductor substrate surfaces. Typically, optical photolithography is achieved by projecting or transmitting light through a pattern made of optically opaque areas and optically clear areas on a mask. The optically opaque areas of the pattern block the light, thereby casting shadows and creating dark areas, while the optically clear areas allow the light to pass, thereby creating light areas. Once the light areas and the dark areas are formed, they are projected onto and through a lens and subsequently onto a substrate.
However, increasing semiconductor device complexity has lead to increased pattern complexity. As a result the pattern packing density on the mask has substantially increased. Additionally, the feature sizes and critical dimensions have steadily decreased, also resulting in denser pattern packing. Consequently, distance between the opaque areas of a mask pattern has decreased. By decreasing the distances between the opaque areas, small apertures are formed which diffract the light that passes through the apertures. Diffraction effects tend to spread or to bend the light as it passes through apertures. This substantially reduces the resolution possible using ordinary masks. This presents an exceptionally severe process limitation for conventional optical photolithography, especially when small features or small geometric patterns are needed.
A number of approaches can be applied to overcome these difficulties. Examples include optical proximity correction (OPC) techniques or the implementation of phase shift masks. This patent concerns phase-shift masks (PSM). A PSM is used in place of conventional masks to image semiconductor surfaces. The operation of a PSM is generally described as follows. In general, light can be thought of as a sinusoidal pattern of light waves that propagate in a medium. “Phase-shifting” describes a change in timing or a shift in the wave form of the regular sinusoidal pattern of light waves. In phase-shift masks, phase-shifting is typically achieved by passing light through areas of a transparent material of either differing thicknesses or through materials with different refractive indexes, thereby changing the phase or the periodic pattern of the light wave. Phase-shift masks reduce diffraction effects by combining diffracted light and phase-shifted light so that constructive and destructive interference takes place. A summation of the constructive and destructive interference results in improved resolution and improved depth of focus.
Two basic types of phase shift patterns can be formed on a mask. One type is an attenuating phase-shift mask. Another type of phase mask is an alternating aperture phase-shift mask (also referred to as a Levenson phase-shift mask). These phase-shift masks are made by a number of conventional methods. An attenuating phase-shift mask typically requires that a layer of opaque material be deposited onto an optically clear plate and then patterned to achieve the required phase-shifting properties. An alternating aperture phase-shift mask (referred to herein as an altPSM or as a Levenson phase-shift mask).
By way of example only, one type of alternating aperture phase-shift mask, as well as a detailed description of theory is disclosed in Marc D. Levenson et al., “Improving Resolution in Photolithography with a Phase-Shifting Mask,” I.E.E.E. TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-29, NO. 12, DECEMBER, 1982 which is hereby incorporated herein by reference.
Such alternating aperture phase-shift masks are commonly made by forming an opaque pattern of material on an optically transparent quartz substrate. The opaque pattern of material defines certain prescribed phase-shift regions in the optically transparent quartz substrate. These phase-shift regions (i.e., the optically transparent regions) are then selectively etched to a pre-determined depth. The pre-determined depth is calculated to induce a desired phase shift in the wavelength of light used in a photolithographic process associated with the mask. Thus, typical methods of making alternating aperture phase-shift masks require extraordinary manufacturing precision. First, the phase-shift pattern must be well aligned to the opaque pattern. Even more importantly, the etching process must be extremely well controlled in order to achieve an etch depth into the quartz to very tightly controlled tolerances. Additionally, such etching must be achieved without leaving significant particle residue. Also, in some implementations the phase-shift regions are formed so that they are slightly larger than the opaque material, thereby creating a phase-shift region that slightly undercuts the overlying opaque areas. This undercut is not only difficult to make and to control, but is also susceptible to particle contamination, thereby ruining mask.
Currently used manufacturing techniques have numerous drawbacks that lead to low yields and long manufacturing process times. This results in the extremely high cost of such alternating aperture phase shift masks. For example, in one conventional approach, dry etch techniques are used to remove material from the quartz substrate in the phase-shift regions. However, such dry etch methods frequently have difficulty obtaining the necessary precision in the final etch profile. This is especially so when many phase-shift regions are to be constructed on a single 130 mm by 130 mm mask without the use of etch stops. Moreover, such dry etch techniques frequently leave an unacceptably high amount of residue and other contamination in the mask, thereby ruining the mask. Wet etch techniques are also used to remove material from the quartz substrate in the phase-shift regions. However, wet etching is an even more difficult process to control and frequently fails to obtain the necessary precision in the final etch depth and profile. The problem is so serious that yields on the order of 10% are not uncommon for existing methodologies. The full extent of these problems are described in numerous publications. For example, Van Den Broeke, Douglas et al., “Transferring Phase-Shifting Mask Technology into Mainstream Technology” Semiconductor Fabtech; Edition 5; October 1996; at http://www.semiconductorfabtech.com/journals/edition.05/index.shtml; which is hereby incorporated herein by reference.
What is needed is a method for fabricating alternating aperture phase-shift masks with a high degree of precision and a relatively high yield.